Web crawler

Web crawler, sometimes called a spider or spiderbot and often shortened to crawler, is an <a href="/facts/Internet_bot/yPTtRLyq">Internet bot</a> that systematically browses the <a href="/facts/World_Wide_Web/wvnTrWFq">World Wide Web</a> and that is typically operated by search engines for the purpose of <a href="/facts/Web_indexing/ege3X1B6">Web indexing</a> (web spidering).
Web <a href="/facts/Search_engine/SRntsbz2">search engines</a> and some other <a href="/facts/Website/SAsjJnca">websites</a> use Web crawling or spidering <a href="/facts/Software/RP79lVSW">software</a> to update their <a href="/facts/Web_content/u4p0t6kz">web content</a> or indices of other sites' web content. Web crawlers copy pages for processing by a search engine, which <a href="/facts/Index_(search_engine)/a0anbq3D">indexes</a> the downloaded pages so that users can search more efficiently.
Crawlers consume resources on visited systems and often visit sites unprompted. Issues of schedule, load, and "politeness" come into play when large collections of pages are accessed. Mechanisms exist for public sites not wishing to be crawled to make this known to the crawling agent. For example, including a <a href="/facts/Robots.txt/VhGtVKEG">robots.txt</a> file can request <a href="/facts/Software_agent/aevgKHF0">bots</a> to index only parts of a website, or nothing at all.
The number of Internet pages is extremely large; even the largest crawlers fall short of making a complete index. For this reason, search engines struggled to give relevant search results in the early years of the World Wide Web, before 2000. Today, relevant results are given almost instantly.
Crawlers can validate <a href="/facts/Hyperlink/LqNsa84e">hyperlinks</a> and <a href="/facts/HTML/sSc3fT78">HTML</a> code. They can also be used for <a href="/facts/Web_scraping/5R4mf1bd">web scraping</a> and <a href="/facts/Data-driven_programming/kpkmDAAp">data-driven programming</a>.

Web crawler open-in-new

Web crawler